The ENR was officially formed 28 October 1943, by merging former Royal Army (Regio Esercito) units still loyal to fascist dictator Benito Mussolini and Italian pro-Nazi units raised by the Germans after the occupation of southern Italy.

On 16 October, the Rastenburg Protocol was signed with Nazi Germany. In accordance with this protocol, the RSI was allowed to raise division-sized military formations. This allowed Graziani to raise four RSI divisions totaling 52,000 men. In July 1944, the first of these divisions completed training and was sent to the front.

Recruiting military forces was difficult for the RSI, as most of the Italian army had been interned by German forces in 1943, many Italians had been conscripted into forced labour in Germany and few wanted to participate in the war. The RSI became so desperate for soldiers that it granted convicts freedom if they would join the army and the sentence of death was imposed on anyone who opposed being conscripted.[1] Autonomous military forces in the RSI also fought against the Allies including the notorious Decima Flottiglia MAS under command of Prince Junio Valerio Borghese. Borghese held no allegiance to Mussolini and even suggested that he would take him prisoner if he could.[1]

During the winter of 1944-1945, armed Italians were on both sides of the Gothic Line. On the Allied side were four Italian groups of volunteers from the old Italian army. These Italian volunteers (of the Italian Co-Belligerent Army) were equipped and trained by the British. On the Axis side were four RSI divisions. Three of the RSI divisions, the 2nd Italian "Littorio" Infantry Division, the 3rd Italian "San Marco" Marine Division, and the 4th Italian "Monterosa" Alpine Division, were allocated to the LXXXXVII "Liguria" Army under Graziani and were placed to guard the western flank of the Gothic Line facing France. The fourth RSI division, the 1st Italian "Italia" Bersaglieri Division, was attached to the German 14th Army in a sector of the Apennine Mountains thought least likely to be attacked.[2]

On 26 December 1944, several size-able RSI military units, including elements of the 4th Italian "Monte Rosa" Alpine Division and the 3rd Italian "San Marco" Marine Division, participated in Operation Winter Storm. This was a combined German and Italian offensive against the American 92nd Infantry Division. The battle was fought in the Apennines. While limited in scale, this was a successful offensive and the RSI units did their part.

In February 1945, the 92nd Infantry Division again came up against RSI units. This time it was Bersaglieri of the 1st Italian "Italia" Infantry Division. The Italians successfully halted the US division's advance. The RSI Minister of Defense, Rodolfo Graziani, was even able to say that he commanded an entire Army. This was the Italo-German Army Group Liguria. However, the situation subsequently deteriorated for the Axis forces on Gothic Line.

On 29 April, Graziani surrendered and was present at Caserta when a representative of German General Heinrich von Vietinghoff-Scheel signed the unconditional instrument of surrender for all Axis forces in Italy. But, possibly as a sign of the low esteem in which the Allies held the RSI, Graziani's signature was not required at Caserta.[5] The surrender was to take effect on 2 May. Graziani ordered the RSI forces under his command to lay down their arms on 1 May.

Dollinger, Hans, The decline and fall of Nazi Germany and Imperial Japan: a pictorial history of the final days of World War II, technical adviser Dr. Hans-Adolf Jacobsen; translated from the German by Arnold Pomerans, Odhams, London 1968 (German edition Die letzten hundert Tage: Das Ende des zweiten Weltkrieges in Europa und Asien, K. Desch, 1965)